This invention relates to a programmable controller with building blocks (hereinafter referred to as PLC of the building block type), provided with device modules with various functions (such as IO module, analog IO module, process IO module, motion control module, process CPU module and communication modules of various kinds supporting communication protocols) such that these device modules can be appropriately selected and combined into a single unit through specified inter-module connection mechanisms in order to flexibly respond to a wide variety of control specifications on the side of the users.
As disclosed in Japanese Patent Publications Tokkai 2007-18503 and 09-181750, for example, there have been known various kinds of PLCs of the building block type provided with device modules with various functions, making it possible to flexibly respond to a wide variety of users' control specifications by combining them into a single unit through specified inter-module connecting mechanisms. Parallel bus, serial bus and bucket relay systems are known examples of method for carrying out inter-module communications in such a PLC of the building block type.
FIG. 10 shows a PLC 2A of the building block type adapted to carry out inter-module communications by the parallel bus method, comprising one CPU module 20 and four device modules 21. The CPU module 20 has incorporated therein a CPU system 201 for carrying out its functions as the central processing unit (CPU) of the PLC, and each of the device modules 21 has incorporated therein a device system 211 which is necessary for carrying out the device functions required of that module.
According to the illustrated example, these modules 20 and 21 are connected together into a single unit through inter-module connection mechanisms of the stacking type. Explained more in detail, these modules 20 and 21 are arranged sequentially in a single line in the right-left direction, with the side surfaces of the mutually adjacent pairs thereof in contact with or near each other and having connectors CN in between. Each connector CN has a left-hand connector half CN(L) and a right-hand connector half CN(R) that together form a pair. The left-hand connector half CN(L) of one of the connectors CN is affixed to the right-hand side surface of the housing of the CPU module 20. The left-hand side surface of the housing of each device module 21 has the right-hand connector half CN(R) of a corresponding one of the connectors CN affixed thereto, and the right-hand side surface of the same housing has the left-hand connector half CN(L) of another connector CN affixed thereto. Each connector CN includes a group of terminals corresponding to signal lines that form a parallel bus PB.
The parallel bus PB, which has its base terminals inside the CPU module 20, is connected to a communication terminal group in the left-hand connector half CN(L) on the right-hand side surface of the housing of the CPU module 20. A communication interface (I/F) 202A is inserted between the parallel bus PB and the CPU system 201.
Inside each of the device modules 21, the communication terminal group inside the right-hand connector half CN(R) on the left-hand side surface of the housing and that inside the left-hand connector half CN(L) on the right-hand side surface of the housing are connected by an internal connector line PCL, and there is a communication interface (I/F) 212A inserted between this internal connector line PCL and the device system 211.
With the modules thus structured, if the housing of each device module 21 is connected to the housing of the CPU module 20 through the connectors CN, the parallel bus PB penetrates through the series of modules 20 and 21, and data of various kinds necessary for the operation of the PLC can be exchanged among the CPU system 201 inside the CPU module 20 and the device systems 211 inside the individual device modules 21.
FIG. 11 shows a PLC 2B adapted to carry out inter-module communications by the serial bus method by using the same symbols to indicate components similar to those described above in FIG. 10. In this example, a bidirectional serial bus SB (including two buses with different directions of data transmission) is connected inside the CPU module 20 to a communication terminal group inside the left-hand connector half CN(L) on the right-hand side surface of the housing, and a communication interface (I/F) 202B with the serial-parallel conversion function is inserted between this bidirectional serial bus SB and the CPU system 201.
Inside each of the device modules 21, the communication terminal group inside the right-hand connector half CN(R) on the left-hand side surface of the housing and that inside the left-hand connector half CN(L) on the right-hand side surface of the housing are connected by an internal connector line SCL, and there is a communication interface (I/F) 212B with the serial-parallel conversion function inserted between this internal connector line SCL and the device system 211.
With the modules thus structured, if the housing of each device module 21 is connected to the housing of the CPU module 20 through the connectors CN, the serial bus SB penetrates through the series of modules 20 and 21, and data of various kinds necessary for the operation of the PLC can be exchanged among the CPU system 201 inside the CPU module 20 and the device systems 211 inside the individual device modules 21 through the serial bus SB.
FIG. 12 shows a PLC 2C adapted to carry out inter-module communications by the bucket relay method by using the same symbols to indicate components similar to those described above in FIG. 10. In this example, a serial bus SB is provided inside the CPU module 20, having its base terminals therein and being connected to a communication terminal group inside the left-hand connector half CN(L) on the right-hand side surface of the housing. Inside the CPU module 20, a communication interface (I/F) 202C with the serial-parallel conversion function is inserted between the serial bus SB and the CPU system 201.
Inside each of the device modules 21, the communication terminal group inside the right-hand connector half CN(R) on the left-hand side surface of the housing is connected to a communication interface (I/F) 212C through a first connection line SCL1, and the communication terminal group inside the left-hand connector half CN(L) on the right-hand side surface of the housing is connected to the communication interface (I/F) 212C through a second connection line SCL2.
With the modules thus structured, if the housing of each device module 21 is connected to the housing of the CPU module 20 through the connectors CN, the serial bus SB penetrates sequentially through the communication interfaces (I/F) 212C of each of the device modules 21. The device system 211 inside each of the device modules 21 will transmit each of the data received from an adjacent module to the other adjacent module in the manner of a bucket relay such that data of various kinds necessary for the operation of the PLC can be exchanged among the CPU system 201 inside the CPU module 20 and the device systems 211 inside the individual device modules 21.
Thus, with a prior art PLC of the building block type, it was a common practice to carry out communications among the modules either through a bus (PB or SB) in multidrop connection by the parallel bus method shown in FIG. 10 or by the serial bus method shown in FIG. 11 or through connection lines (SCL and SCL2) between adjacent pairs of modules by the bucket relay method shown in FIG. 12.
If communications between the modules are by the parallel bus method shown by FIG. 10, the communication speed cannot be improved due to skews among the bits, cross talks and ground bounce. If the communications are by the serial bus method shown by FIG. 11, there is also a limit to the improvement in the communication speed because the effects of signal degradation due to 1-to-N connections and ground bounce are not negligible. As for the communications among the modules by the bucket relay method, there is the problem of all modules becoming incapable of carrying out communications because of a trouble in one of the intermediate modules. By any of these methods using a bus or connection lines among the modules, there is the problem of the transmission capability being significantly affected as the number of the connected modules is increased because of the limitations arising from the basic structure of the method using a bus.